This invention relates to a hydraulic drive system for actuating a single rod cylinder, including a closed hydraulic circuit having a hydraulic pump and connected to the cylinder, and more particularly, to a hydraulic drive system equipped with a flushing valve for discharging from the closed hydraulic circuit excess fluid produced therein when the single rod cylinder is actuated.
To actuate a single rod cylinder, a closed hydraulic circuit has been proposed which includes a hydraulic pump, a main line for communicating one port of the hydraulic pump with a rod side port of the single rod cylinder, and another main line for communicating another port of the hydraulic pump with a bottom side port of the single rod cylinder. When a piston rod of the single rod cylinder is withdrawn into the cylinder, the fluid flowing into the cylinder through the rod side port is smaller in flow rate than the fluid flowing from the cylinder through the bottom side port, thereby causing excess fluid to be produced in the closed hydraulic circuit. To discharge the excess fluid from the closed hydraulic circuit, a flushing valve is used which includes two inlet ports respectively connected to the two main lines and one outlet port connected to a fluid tank. In the flushing valve, communication between the two inlet ports and the one outlet port is normally blocked. However, when a predetermined pressure differential is produced between the two main lines, the inlet port connected to the main line of lower pressure is brought into communication with the outlet port, to thereby allow the excess fluid in the closed hydraulic circuit to be returned to the fluid tank.
However, the above mentioned hydraulic drive system utilizing the closed circuit cannot be used to actuate a single rod cylinder connected to an element which reverses a direction of load applying on the cylinder during movement thereof, such as, for example, a shovel or an arm in an earth-moving machine or a construction machine. Suppose that the single rod cylinder is being actuated to move the piston rod into the cylinder by a high pressure fluid from the hydraulic pump. At this time, the main line connected to the bottom side of the cylinder is lower in pressure than the other main line, and the flushing valve is in a position in which it allows the bottom side main line to be connected to the fluid tank, so that the excess fluid is drained from the bottom side main line through the flush valve to the fluid tank. Under such conditions, it may sometimes happen that the direction of a load driven by the single rod cylinder is suddenly reversed so that the single-rod cylinder which has driven the load is driven by the load in the direction in which the piston rod moves into the cylinder. When this occurs, the bottom side main line that has been lower in pressure has its pressure increased while the pressure in the rod side main line is decreased, so that the flushing valve is switched through a neutral position to a position opposite to the position in which it was located. Upon the flushing valve reaching the neutral position, however, the two inlet ports are brought out of communication with the outlet port, so that the excess fluid in the closed hydraulic circuit has nowhere to go. This brings the single rod cylinder to an abrupt halt, thereby causing an inordinately high pressure to be generated in the closed hydraulic circuit and giving a shock referred to as a lock-up phenomenon to the hydraulic drive system. As can readily be appreciated a lock-up phenomenon should be avoided.